The present invention generally relates to adhesives for joining wood products, and more particularly to two-part polyurethane adhesives modified with soy protein which adhesives are both heat and radio frequency (RF) curable.
Glued wood products have been traditionally used in this country in a variety of applications. The adhesives for such bonded or laminated wood products conventionally are based on phenol-formaldehyde, urea-formaldehyde, polyvinyl acetate, resorcinol-formaldehyde, polymeric diphenylmethane diisocyanate (MDI), and hot met adhesives. The glued wood products include plywood, particleboard, oriented strand board (OSB), medium density fiberboard (MDF), laminated veneer lumber (LVL), laminated beams, and a variety of other engineered wood products. Among them, laminated beams, I-beams, LVL, and a variety of engineered wood products are used for structural wood applications. Generally, these engineered wood products require an initial finger jointing of short pieces of wood or parallel laminated veneers (PLV) before they can be constructed into long and/or thick beams or lumbers. Consequently, it is important that the finger-jointed area must have good strength to be used for structural wood applications. For present purposes, all of the foregoing products are known as "laminated wood products."
At present, phenol-resorcinol-formaldehyde (PRF) is widely used in industry for finger joint applications. When adhesive is applied to the fingers, the finger jointed wood or PLV is crowded together using an end pressure until a "tip gap" of 1-40 mils is achieved. Its is essential that the fingers do not "bottom out." The finger joints then are moved into a curing zone where hot platens or dielectric plates are used to cure the finger joints under heat or radio frequency and pressure for typically less than 30 seconds and then the joints are removed away from the curing zone. The adhesives must be able to fill the gaps or voids between the fingers when curing is complete in order for the product to exhibit good strength and a smooth appearance.
One the other hand, the speed of curing must be fast under these conditions in order to hold the finger-jointed pieces together for further processing, such as beam lamination and I-joint assembly. This is especially true in a high-speed commercial finger jointing process. Generally, adhesives with high solids and fast curing profiles are regarded as appropriate for such an application.
It should be mentioned also that two-part PRF and melamine-formaldehyde (MF) adhesives generally are used in industry as adhesives in preparation of radio frequency (RF) cured finger joint assemblies. Both of these adhesives are highly polar, which makes them respond well to RF curing. RF curing drives the bondline temperature sufficiently high to permit the MF or PRF to condense quickly. Due to the rigid ring structure of both PRF and MF, these adhesives cure to give a high T.sub.g, rigid, three-dimensional network in a short time; thus, producing acceptable immediate handling strength (proof-loading).
Soy protein products have been utilized as adhesives for wood bonding since the 1930's. Their use soon declined as a result of the development of petroleum derived adhesives. Economic and ecological pressures in the 1990's renewed interest in the practical use of soy protein products in wood adhesives. Heretofore, soybean protein has been proposed for use with phenolic resin, urea resin, or resorcinol resin, such as is described in JP 06200226, 58034877, 50034632, and 04057881. In JP 50034632 and 04057881, adhesives are proposed that consist of soy protein as the major component and isocyanate as a minor component because the soy protein, like wheat flour, is capable of thickening the mixture by absorbing moisture in the wood to give an adhesive with fast drying capability. In recent years, soybean-based adhesives were developed and used primarily in a "honeymoon" system, such as those described by Scheid, "Finger-Jointing and Other Uses of Environmentally Friendly Soy-Based Adhesives", Wood Technology Clinic and Show, Conference Proceeding, Portland, Oreg., Mar. 25-27, 1998; and Steele, et al., "Finger Jointing Green Southern Yellow Pine With A Soy-Based Adhesive", Wood Technology Clinic and Show, Conference Proceeding, Portland, Oreg., March 25-27, 1998.